During the manufacture of certain products, such as electrical components, it is necessary to be able to provide high-intensity illumination so that components can be thoroughly inspected with a machine-vision system. Often, the light source needed includes one or more light sources, for example a ring-shaped flashtube or a number of light-emitting diodes arranged along a circle or a remote light source that drives light into a number of optical fibers arranged along a circle, surrounding the lens of a video camera such that the object being imaged by the video camera is illuminated with light angled in towards the optical axis of the camera from the light source surrounding the lens. It is desirable that the light source or sources are arranged such that no light shines directly from the light sources into the lens.
Typically, a xenon flashtube or laser-based single-point source or other high-intensity light source is used for providing light into fiber-optic-based ring source. Such systems, however, are costly, very large and bulky, and can interfere with the placement of other components in the machine-vision system. This is particularly troublesome when the components being measured or inspected are extremely small. Xenon flashtube light sources also tend to exhibit up to about a five per cent (5%) flash-to-flash variation in intensity which makes accurate measurements of certain characteristics difficult. Single-point source systems are also generally limited to emitting light radially from only one single point, which is of limited value when shadows are problematic, such as, when inspecting a grid of electrical connectors. Specifically, light from only one or just a few point sources only illuminates the first over-sized or over-height electrical connector and, due to shadows from the first object encountered, does not provide proper illumination which would determine if other objects behind this particular first object are missing, of the incorrect size or height, or perhaps in the wrong position.
Conventional illumination systems produce a light which can be too bright in certain areas and too dim in other areas. Often, the end-result is "bloom", especially when viewing white, lightly colored, or very reflective objects which are near other objects which need to be viewed by a machine-vision camera. In order to get enough light on the other objects which need to be viewed, the aperture on the camera cannot be "stopped down" in order to prevent overexposure of the bright objects. Specifically, the area is illuminated to such an extent that the entire image appears to be the same bright saturated white color (or, if a monochromatic light source is used, saturated at whatever color is used) as viewed by the machine-vision camera and system. Such extreme brightness also poses a danger of blinding, at least temporarily, human workers nearby.
Quite often, illumination sources either leave certain portions of the scene in shadows, or provide too much light in certain areas, while leaving other areas with too little light. In other cases, the illumination source is too bulky and gets in the way of other components of the machine-vision system, associated robots, manipulators, and/or human workers.
The optimal light-source-to-optical-axis angle can vary depending on the object being inspected. One shortcoming of conventional ring light sources is the cost and difficulty in changing the angle between the light sources relative to the optical axis, and in changing the spread and/or focus of the light from ring-light source.
Thus, what is needed is an ring-light illumination system and method which is compact, provides control over both the angle between the light source and the optical axis of the camera, as well as the spread and/or focus of the light from ring light source, so that even extremely small parts can be quickly and adequately inspected and accurately viewed or measured with a machine-vision system. Another need is to provide a compact illumination source, preferably monochromatic, which can be focused to provide uniform multi-directional light onto objects from all sides while avoiding light going directly from the light sources to the lens of the camera. Another need is to provide a compact monochromatic LED (light-emitting diode) illumination source, which can be changeably focused to provide uniform multi-directional light onto objects. Another need is to have such an LED illumination source be pulsed with a relatively high-power, low duty-cycle power source.